Anupam, try B&H. Search for step ring, then general brand. They start with 25.5 mm to something. Also try www.srbfilm.co.uk , they start at 25.0
I usually get mine from a fellow named John Aungst, who brings a mountain of filters and step rings and such to the 2nd Sunday Camera Show in Wayne, NJ. The advantage of dealing with Mr. Aungst is that I can take the item and try it out with various rings. I believe that he has a photo studio in Reading, PA, use Google, whowhere.com, yellowpages.com, and so on to look for him.